1. Field of the Invention
The present invention relates to a light-emitting device.
The present invention further relates to a method of manufacturing a light-emitting device.
2. Related Art
Organic Light Emitting Diodes (OLEDs) are the basis for a new display and lighting technology. OLED devices include a thin film of electroluminescent organic material sandwiched between a cathode and an anode, with at least one of these electrodes being a transparent conductor. When a voltage is applied across the device, electrons and holes are injected from their respective electrodes and recombine in the electroluminescent organic material through the intermediate formation of emissive excitons.
A bottom emitting OLED may be thought of as consisting of a core containing high index of refraction layers (organic layers for light generation, carrier transport, injection or blocking, and, typically, a transparent conductive oxide layer) and a low index of refraction substrate material, for example a glass plate or a polymer film. Therefore light that is generated within the core may encounter high index to low index interfaces where it might undergo internal reflection. Light unable to escape the core as a result of encounter at the first interface is confined to a waveguide mode, while light passing through that interface but unable to escape from the substrate as a result of reflection at the substrate-to-air interface is confined to a substrate mode. Similar optical losses occur due to interfaces in top emitting OLEDs.
In this way in OLED devices, over 70% of the generated light is typically lost due to processes in an evanescent zone close to the electroluminescent organic layer and the substrate air interface. The majority of the light undergoes internal reflections, which result in its being emitted from the edge of the device or trapped within the device and eventually being lost to absorption within the device after making repeated passes.
Solutions to improve the out-coupling of light at the substrate to air interface are quite common and rely on scattering or (micro) lenses at the substrate surface.
Solutions to improve the coupling of light from the OLED into the substrate are less common. Various solutions have been proposed to affect light reaching the substrate by disturbing that interface. In the evanescent zone, the exponentially decaying electromagnetic field in this zone is also known as evanescent field. Only part of the emitted light emerges through the transparent electrode as “useful” light into the substrate.
WO 2009011961 discloses a method for making an optical film for enhancing light extraction. The method comprises the steps of                coating a layer of an organic material having a first index of refraction onto a flexible substrate;        imparting nanostructured features into the organic material to create a nanostructured surface; and        applying a backfill layer to the nanostructured surface to form a planarizing layer on the nanostructured surface, wherein the backfill layer comprises a material having a second index of refraction different from the first index of refraction, and wherein a substantial portion of the nanostructured features are within an evanescent zone adjacent to a light emitting region of a self-emissive light source when the optical film is located against the self-emissive light source.        
The optical film, so obtained is used to enhance light extraction in an OLED. The OLED comprises a self-emissive light source having at least one surface that outputs light from the device; and a light extraction film adjacent the at least one surface of the self-emissive light source, wherein the light extraction film comprises: a flexible substrate; a structured layer of extraction elements having a first index of refraction, wherein a substantial portion of the extraction elements are within an evanescent zone of the light output surface of the self-emissive light source; and a backfill layer comprising a material having a second index of refraction different from the first index of refraction, wherein the backfill layer forms a planarizing layer over the extraction elements, wherein the structured layer and backfill layer are in sufficient proximity to the light output surface of the self-emissive light source in order to at least partially enhance the extraction of light from that surface. In the device known from WO 2009011961 the backfill layer is arranged at a side of the structured layer that faces the OLED surface.
Necessarily with the optical film manufactured in this way the distance from the surface of the OLED to the interface between the structured layer and the backfill layer is still relatively large. In order to provide for a sufficient planarization the backfill layer should have a relatively high thickness.
WO 2009011961 remarks “When the backfill layer has a lower index than the structured layer, then the backfill layer preferably has a thickness substantially equal to the extraction elements. When the backfill layer has a higher index than the structured layer, then the backfill layer can be thicker than the extraction elements provided it can still interact with the evanescent wave. In either case, the structured layer and backfill layer are preferably in sufficient proximity to the light output surface in order to at least partially effect the extraction of light from that surface.”
WO 2009011961 further notes that additional functionality could be incorporated into the light extraction film product by forming the extraction structures on an optional ultrabarrier film, which provides excellent moisture and oxygen barrier properties. Ultrabarrier films include multilayer films made, for example, by vacuum deposition of two inorganic dielectric materials sequentially in a multitude of layers on a glass or other suitable substrate, or alternating layers of inorganic materials and organic polymers, as described in U.S. Pat. Nos. 5,440,446; 5,877,895; and 6,010,751, all of which are incorporated herein by reference.
There is a need for further improvement of the extraction efficiency of OLED devices.